Method of cakburizing



July 13, 1943- E. F. DAVIS METHOD OF CARBURIZING Original Filed Jan. 5, .1940

Reissued July 13, 1943 METHOD OF CARBURIZING Ernest F. Davis, Muncie, Ind, assignor to Borg- W'arner Corporation, Chicago, 111., a corporation of Illinois Original No. 2,275,133, dated March 3, 1942, Seria1 N0. 312,020, January 2, 1940.

Application for reissue April 30, 1942, Serial No. 441,147

3 Claims.

This invention relates to the carburizing of gears and other articles of steel having bores,

and has as its object to provide a method whereby the teeth and other external areas of the gears may be hardened while the bores are maintained in an annealed state.

More specifically, the invention contemplates a carburizing method wherein the bores of the articles are packed or lined with a material adapted to inhibit the carburizing action in the vicinity of the bore surfaces, without affecting the carburizin action on the external surfaces.

One of the problems that has been encountered in the development of the invention is that of confining the inhibiting effect to the regions that are to be maintained in an annealed condition. An important object of the invention, therefore, is to provide a method whereby the external surfaces may be fully and uniformly hardened while the hub bores are maintained in a soft condition.

Other objects, the advantages and uses of the invention will become more apparent after reading the following specification and claims, and after consideration of the drawing forming a part of the specification, wherein:

Fig. l is a schematic sectional view of a carburizing apparatus in which the method of the invention is illustrated as being practiced;

Fig. 2 is a detailed sectional view of a portion of one of the article holding trays showing the articles stacked thereon preparatory to carburizing; and

Fig. 3 is an enlarged detailed sectional view of a portion of the sleeve of inhibitin material which is employed to line the bores of the gear hubs in order to inhibit the carburizing effect therein.

In the practice of the invention, I utilize a standard carburizing furnace which may include a combustion chamber In muffle l l and the outer housing l2 of the furnace. With the muffle I I there is maintained an atmosformed between a phere l3 of carburizing fluid which is recirculated by means of a pump 14, being drawn from the forward region of the muffle through a pipe l5 and returned to the rearward region of the muflle through a pipe IS,

The carburizing atmosphere may comprise from 30% to 60% of carburizing gas, preferably in the neighborhood of 30% to 35%, from 40% to 60% hydrogen, preferably in the neighborhood of 54%, and the remainder impurities, largely composed of nitrogen entering the muflie from the external atmosphere during the periods when the end closures are open. The carburizing gas may be any gaseous hydrocarbon, such as methane, propane or butane, methane being preferred. The carburizing agent may also be liquid or solid hydrocarbon, for example, the type used in conventional case hardening operations.

The respective ends of the muffle II are closed by closure members I1 and I8 respectively which may be conventional vertically slidable doors adapted to be raised to allow the entrance at one end and the exit from the other end of the trays I9 and the gears 20 stacked thereon. The atmosphere l3 within the muffle is maintained at carburizing temperatures (in the neighborhood of l700) by combustion within the chamber I0 of a suitable fuel, preferably hydrocarbon gas, which may be introduced through fuel pipes 21' and 22 leading through the housing l2. In such atmosphere, the steel gears 20 absorb carbon from the carburizing gas, liberating hydrogen which is disposed of by combustion at the mouth of the mufile when the doors I! and iii are raised. Fresh carburizing gas may be introduced into the muffle to take the place of that which is consumed, by means of a charging tube 23 communicating with the tube l6 through the medium of a suitable regulating'valve 24.

In accordance with the present invention, the bores 32 of the gears 20 are maintained in a su liciently annealed condition to permit of e; machining, by inhibiting the action of the carburizing agent in the vicinity of the bores. To this end, the invention employs an inhibitor in the form of an oxygen bearing substance adapted to combine with the carbon in the carburizing agent so as to form a blanket of carbon dioxide gas protecting the surfaces of the hub bores against the effect of the carburizing agent.

The invention contemplates the employment of an inhibitor in the form of an oxide wherein the oxygen has a greater afiinity for the carbon of the carburizing agent than it has for the element with which it is originally in combination. Suitable for this purpose are reducible metallic oxides capable of remaining in a solid condition at carburizing temperature. Specific examples of me tallic oxides which may be successfully employed are iron oxide and copper oxide.

Iron. oxide is preferred, and the preferred form thereof is forging scale.

In preparing the gears for carburizing, they are first threaded onto a core member 21 in the form of a sleeve having at its lower end a flange 28 upon which the lower gear is adapted it) rest, threaded as at 29 at its upper end and provided with a nut 30 by means of which the stack of gears is secured upon the sleeve.

The stacks of gears are mounted upon trays ll having posts 3| over whichthe sleeves 21 are slipped.

The external diameter of the sleeve 21 is con siderably less than the internal diameter of the bores 32 of the gear hubs 33 and as the gears are threaded onto the core sleeve 21, the annular space between the sleeve and the bores 32 is filled with the inhibitor mixture 34 in the form of a sleeve lining the bores of the hubs.

It will now be apparentthat as the stacks of gears are passed through the carburizing atmosphere, any carburizing gas seeping between the hubs 33 will have to pass into and through, the sleeve 34 of inhibitor material in order to reach I the invention.

Substantially, the iron may be considered as an inert material functioning as a diluent, and the invention contemplates the use of other substances which are inert at carburizing temperatures, as, for example, sand or comminuted glass.

The mixture of iron cuttings and scale is shown in Fig. 3, the iron cuttings being indicated at a 25 and the scale at 28.

bu rizing agents, the spacing between stacks of,

gears, the rapidity of movement of the carburizing atmosphere, etc. The invention aims to obtain a balance between the oxide and the inert the bores 32 of the hubs. In thus passing through material into the space between the core sleeve 21 and bores 32, the space should be in the neighborhood of V inch thick.

The unadulterated iron oxide has proven to be satisfactory in the presence ,of some carburizing mediums. But in the carburizing of gears with a gaseous carburizing agent, I have experienced considerable difilculty with undercarburizing or uneven carburizing of the gear teeth. After contending with the problem for some time it was finally decided that the diiliculty arose from the production of an excess volume of carbon dioxide within the bore of the gear, and the forcing of such excess outwardly through apertures between the gears formed by irregularities in the surfaces thereof, thus partially blanketing the teeth.

In attempting to confine the action of the inhibitor to the region of the bores of the hub members, it is not practicable to attempt to prevent the escape of excess carbon dioxide. In order to meet the problem, the invention reduces the inhibiting effect by diluting the oxide with a quantity of non-oxidizing material. The material preferred for this purpose is a material which will not react with the metal of the gear to any substantial extent, as, for example, iron in comminuted form, preferably milling machine cuttings. Such cuttings, as well as forging scale, are available in the average machine shop as waste products. The cuttings are preferably from a steel of low carbon content.

The iron filings present a large surface upon which any carburizing effect that the gas may be able to develop over the inhibiting effect of the iron oxide, will be largely dissipated before the gas comes in contact with the bore of the gear. Thus the iron may be effective to reduce to some extent the activity of the carburizing gas within the bore without devoloping CO or C02. However, the comminuted iron alone, without the iron oxide, will not satisfactorily achieve the object of material, wherein there is just suflicient oxide to inhibit carburizing within the bores of the gears but not enough to throw out an excess of C02.

Best results have been secured by employing about 50% oxide and 50% diluent. Good results maybe obtained, however, within'the range from 80% diluent and 20% oxide to 25% diluent and omde.

The work is maintained in the carburizing atmosphere a normal carburizing period, which may'range from 14 to 16 hours, and is then discharged and lowered into a quenching bath, whence it is carried through a cleaning bath and a tempering furnace.

I claim:

1. The steps in the method of carburizing articles of steel having a bore, said articles being stacked together so that their bores collectively form a chamber, which method includes: introducing into the said chamber a mixture of comminuted iron and iron oxide in the proportion of to 25% iron and 20 to 75% iron oxide, providing closure means closing said chamber, and maintaining such mixture in contact with the surfaces of the bores while passing the articles through a carburizing atmosphere.

2. The steps in the method of carburizing articles of steel having a bore, said articles being stacked together so that their bores collectively form a chamber, which method comprises introducing into the said chamber a mixture of approximately 50% comminuted iron and 50% iron oxide in the form of forging scale, providing closure means closing said chamber, and maintaining such mixture in contact with the surfaces of the bores to inhibit the carburizing effect on said surfaces while passing the articles through a carburizing atmosphere.

3. The steps in the method of carburizing articles of steel having a bore, said articles being stacked together so that their bores collectively form a chamber, which method includes introducing into said chamber a mixture of comminuted iron and iron oxide in a proportion of 80 to 25% iron and 20 to 75% iron oxide, providing closure means closing said chamber, and maintaining such mixture in proximity to the surfaces of the bores while passing the articles through a carburizing atmosphere.

. ERNEST F. DAVIS. 

